For every live being, ecological conditions greatly influence biology. In the case of a fish, these conditions may be responsible for its presence or absence in the medium, its development and its population size. This work studies the characteristics and physicochemical typology of Sidi Rachid River, known by its wealth of the Salmonidae fish: Salmo trutta macrostigma (Duméril, 1858). The results show that apart from the appearance of nitrates, whose high levels indicate an organic pollution, during some times of the year, coming seemingly from the Ras Elma fish farm waste water, the water quality is good. The results also show that the physico-chemical typology of the watercourse is mainly influenced by the "season" factor and that among the studied variables, only the pH does not influence the spatial and temporal variation of the medium studied characteristics.Key words: Sidi Rachid River, physicochemical typology, Salmo fario, Morocco.